The present invention relates to a device for locking a platform of an offshore structure so that the platform may securely rest on supporting legs.
In order to tap submarine oil reservoirs or to accomplish various offshore operations, an offshore structure 1 called jacked-up rig as shown in FIG. 1 is used. The jacked-up rig 1 comprises a plurality of upright or vertical legs 3 resting on the sea floor 2 or the like and a platform 4 resting on the legs 3 and capable of being jacked up. Utilities and a living quarter are installed on the platform 4.
In operation, the rig 1 has its platform 4 securely rested on the legs 3 while when the height of the platform 4 above the sea level is varied or when the rig 1 is floated and towed, the platform 4 is lifted or lowered in the longitudinal direction (that is, in the vertical direction) of the legs 3. As shown in FIG. 2, a rack 5 disposed in the lengthwise direction of the leg 3 is in mesh with a pinion 6 disposed on the side of the platform 4 so that as the pinion 6 rotates, the platform 4 is raised or lowered relative to the legs 3.
In operation, the rig 1 is subjected to various external forces such as forces of waves, winds and currents so that the platform 4 must rest on the legs 3 as securely as possible. To this end, in addition to the above-described rack-and-pinion type jack, the rig 1 is provided with a locking device for securely supporting the platform 4 upon the legs 3.
One of the conventional locking devices is shown in FIG. 2. The platform 4 is provided with a chock 7 which is adapted to engage with the rack 5 on the side of the leg 3. The engagement between the chock 7 and the rack 5 can support the weight of the platform 4 and withstand the external forces such as the forces of waves, winds and currents. The chock 7 can be raised or lowered by means of two wedges 9 with their inclined surfaces 8 in contact with upper and lower surfaces of the chock 7. An actuator 10 is securely joined to a rear surface 7a of the chock 7 for the movement of chock 7 toward or away from the rack 5.
In order to engage the platform 4 with the legs 3, the pinion 6 is driven until the platform 4 reaches a predetermined height. Thereafter, drive means (not shown) is energized to horizontally slide the wedges 9 in opposite directions, whereby the chock 7 is raised or lowered for alignment of the teeth. Next the actuator 10 is energized to engage the chock 7 with the rack 5. Under these conditions, the wedges 9 are displaced toward the rack 5 and the pinion 6 is released so that it may freely rotate. Thus, the platform 4 can rest on the legs 3 as the load of the platform 4 is supported by the teeth of the chock 7 and rack 5 which are engaged with each other.
In the conventional locking device as described above, as shown in FIGS. 3A, 3B and 3C, the teeth 11 of the rack 5 and the teeth 12 of the chock 7 have the same configuration so that they may mesh very closely. These teeth 11 and 12 are not produced by machining, but are formed by a gas cutting process or the like so that they tend to have pitch errors. When the rack 5 and the chock 7 are engaged with each other so that the platform 4 may be supported by the legs 3, the weight of the platform 4 and the external forces such as forces of winds, waves and currents are exerted to the teeth 11 and 12. As a result, the teeth 11 and 12 are forcedly deflected. According to the experiences, it has been known that the deflection is substantially equal to the pitch error. This will be described in more detail below. As shown in FIGS. 3A through 3C, the fact that the platform 4 is supported by the legs 3 means that the teeth 12 of the chock 7 are supported by the teeth 11 of the rack 5. As a result, both the teeth 11 and the teeth 12 are deflected. As shown in FIG. 3A, even when all the teeth 11 and 12 have exactly a predetermined pitch P, all the load of the platform 4 cannot be uniformly distributed to the teeth 1 of the rack 5. There is a tendency that the load is concentrated on the uppermost and lowermost engaged teeth 11 of the rack 5. If the pitch of some tooth 11 of the rack 5 is greater than a predetermined pitch P and for example is (P+P.sub.1) as shown in FIG. 3B, said tooth 11 and the corresponding tooth 12 are spaced by a very small distance as if they were in abutment with each other. Then, a higher load is exerted to said teeth 11 and 12 so that the deflection .delta..sub.L occurs. On the other hand, if the pitch of some tooth 11 of the rack 5 is smaller than a predetermined pitch P and for example is (P - P.sub.2) as shown in FIG. 3C, no load is exerted to said tooth 11 and the corresponding tooth 12 which are spaced apart from each other by a distance equal to or greater than the deflection .delta. to be expected in the engagement of the teeth with no pitch error.
It is considered that when the teeth of the chock and the rack have the same configuration, they are subjected to load concentration (as shown in FIG. 3B) or no load is exerted to the teeth 11 and 12 (as shown in FIG. 3C). Such loading conditions are not satisfactory from the viewpoint of mechanical strength. In order to overcome these problem, it may be proposed to increase the size of the teeth and hence their strength, but the legs 3 and the chocks 7 will then become heavy in weight and expensive. Thus, such proposal is not satisfactory.
The present invention was made to overcome the above and other problems encountered in the conventional devices for locking a platform of an offshore structure and has for its object to provide an improved device for locking a platform of an offshore structure in which each of the teeth of racks and chocks can uniformly bear the load of the platform and external forces such as waves, winds and currents, whereby the mechanical strength and safety of the locking device may be improved.
The above and other objects, effects and features of the present invention will become more apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.